1. Field of the Invention
The present invention relates to an improvement in technique for slicing a workpiece, such as a semiconductor silicon ingot, to obtain wafers.
2. Description of the Related Art
Conventionally, there has been known a method of slicing off wafers from a workpiece such as a semiconductor silicon ingot through use of a workpiece slicing apparatus, i.e. a wire saw. In the wire saw, a fine steel wire is spirally wound and extended among a plurality of rollers such that turns of the wire are equally spaced in the longitudinal direction of the rollers. The wire is fed from a wire feed side to a wire take-up side with or without reciprocal movement. A workpiece is pressed against the thus-fed wire and fed at a predetermined rate while abrasive grain slurry is fed to the press contact portion between the wire and the workpiece, whereby a plurality of wafers are slicing off concurrently. The abrasive grain slurry is a suspension of fine abrasive grains in oil or water-soluble coolant. Since this method can be used to concurrently slice off a number of wafers, it is becoming increasingly popular, replacing a conventional method that employs an inner diameter slicer.
In the method of slicing a workpiece through use of a wire saw, the feed rate of a workpiece is generally held constant or controlled according to a program for maintaining a constant cutting load to the extent possible. The cutting load is a load imposed on a wire 1 when a workpiece W is cut by the wire 1 (FIG. 1). When the workpiece W having a circular cross section is cut, the cutting load is equivalent to the product of the chord length of cut as shown in FIG. 1A and the feed rate of the workpiece W as shown in FIG. 1B. The feed rate of the workpiece W is controlled such that the cutting load becomes as constant as possible, as shown in FIG. 1C.
Abrasive grains contained in abrasive grain slurry effect an actual action of grinding a workpiece when the workpiece is sliced by a wire saw. As the size of abrasive grains decreases due to wear or fracture of abrasive grains themselves, the slicing stock removal decreases; as a result, the thickness of a wafer obtained by slicing increases. As shown in FIG. 2, the thickness of a wafer having a diameter of 200 mm shows a tapered variation of 8 .mu.m to 10 .mu.m as measured after slicing, discounting the wafer's slicing start and end portions, which correspond to approximately 1/10 of the overall depth of cut and at which locations the thickness of the wafer varies significantly. As shown in FIG. 2, in the slicing start and end portions of a wafer, the thickness of the wafer varies significantly since a sudden change in cutting load induces a change in deflection of a wire and a change in wear of the wire itself, or a change in the amount of abrasive grain slurry brought from the wire into a cutting groove.
The thicknesswise taper of a wafer obtained by slicing is difficult to eliminate in the subsequent lapping, etching, and polishing processes. Thus, there has been an eager demand for a method of slicing off wafers through use of a wire saw in such a manner that the degree of the thicknesswise taper of each wafer obtained by slicing is small to thereby provide wafers having an improved uniform thickness.